Magnetic track following servo system



Oct. 1, 1968 F. J. SORDELLO I 3,404,392

MAGNETIC TRACK FOLLOWING SERVO SYSTEM Original Filed Dec. 18. 1962 DATADATA SERVO SERVO SERVO POSITION SERVO SIGNAL VOLTS v 2 SERVO TRACK F 3SERVO TRACK F i 600 F2 EVEN F1 DATA TRACK P [PM DISK CROSS SECTIONHFDATA M16 FILTER AMP PA FILTER LINEAR I PEAK AMP GATE RECT wmE 11 19 212s Dc BND SUMMING 7 AMP FZFILTER u AR 825 N PEAK 2 AMP GATE RECT A4 a eADDRESS REGISTER 5M ACTUATOR INVENTOR.

H FRANK SORDELLO ATTORNEY United States Patent 3,404,392 MAGNETIC TRACKFOLLOWING SERVO SYSTEM Frank J. Sordello, San Jose, Calif., assignor toInternational Business Machines Corporation, New York, N.Y., acorporation of New York Continuation of application Ser. No. 245,572,Dec. 18, 1962. This application Apr. 14, 1967, Ser. No. 631,103 4Claims. (Cl. 340-174.1)

ABSTRACT OF THE DISCLOSURE A servo positioning system for accuratelypositioning a transducer over a data track by detecting a null zonebetween two adjacent servo tracks. Two magnetic layers of differentcoercivity are used such that a plurality of continuous servo tracks arerecorded in the lower layer while data tracks are recorded in the upperlayer, centered over the null zone between adjacent servo tracks. Asingle read gap magnetic head is used for simultaneously reading signalsfrom both servo tracks in the lower magnetic layer and the data trackfrom the upper magnetic layer.

This is a continuation of copending application Ser. No. 245,572, filedon Dec. 18, 1962, for Magnetic Track Following Servo System, and nowabandoned. This invention relates to a servo system and moreparticularly to a track following servo system for positioning atransducer at a desired track location on a magnetic recording medium.

In disk-type random access magnetic memories where data is recorded inconcentric circular tracks on the surfaces of disks it is a continuingaim to accurately align a magnetic transducer with a desired track. Thedegree of accuracy with which the transducer can be positioneddetermines the spacing necessary between adjacent tracks and therebylargely influences the storage efficiency, i.e., number of charactersper unit of area of the memory. In an attempt to increase the accuracyof alignment, servo systems of various types have been proposed forservoing the transducer onto the tracks. These systems have generallyemployed positioning information in the form of servo signalsinterspersed with the data in the recording surface or referencepatterns permanently recorded on a disk surface. In addition, suchsystems have required a servo transducer to read the positioninginformation and a separate data transducer ganged thereto. Thesefeatures of the known servo systems inherently militate against highstorage efficiencies because of the stack-up of mechanical tolerances inthe ganged transducers and the fact that a considerable portion of theavailable disk surface area is given over to the storage of positioninginformation.

An object of the present invention is to provide a track following servosystem for a random access magnetic memory to maintain a transducer inaccurate alignment with a recording track, thus permitting a highstorage efficiency for the memory.

The above object is realized in the present invention by the provisionof a system for servoing a transducer into alignment with a desired datatrack on a magnetic recording medium. A single continuous linearlyrecorded servo track is located between each pair of adjacent datatracks, alternate servo tracks being written at different frequencies. Asingle transducer is provided for simultaneously reading a data trackand the servo tracks on either side thereof, and means is provided forfiltering the data from the servo information and then comparing the twoservo signals to develop a position error signal for the transducer. Theerror signal is then supplied to an actuator to position the transducer.

The foregoing and other objects, features and advan- 3,404,392 PatentedOct. 1, 1958 tages of the invention will be apparent from the followingmore particular description of a preferred embodiment of the invention,as illustrated in the accompanying drawing, wherein:

FIG. 1 is a block diagram of circuitry for positioning a transducer overa desired data track on a magnetic recording disk;

FIG. 2 is a plan view illustrating the servo tracks employed in thepresent system and their relation to the data tracks;

FIG. 3 is a plot of the servo outputvoltage variation with transducerposition;

FIG. 4 is a cross section of a magnetic recording disk;

FIG. 5 is a composite of FIGS. 5a and 5b;

FIG. 5 a] is a side cross section or" a transducer and erasing head; and

FIG. 5b is a bottom view of a transducer and erasing head.

As shown in FIG. 1, a transducer 10 is positioned on a magnetic disk 11by an actuator 12. Coarse positioning signals are transmitted from atrack address register 13 to the actuator to position the transducerover a desired track on the disk. Fine positioning signals are thendeveloped in the circuitry of FIG. 1 to control the actuator to keep thetransducer centered over the middle of the desired track.

To provide a track following servo, the transducer position error signalmust show a measure of the distance off track and a sense or signindicating direction. In addition this position characteristic shouldhave a null point where the data signals are to be written. Toaccomplish this, servo signals may be written on either side of the datatrack, so that the data track lies exactly between the servo signals.The servo signals are written such that they are read back with equalamplitude when the head is directly centered on the data track betweenthem. Provision can be made to take the difference of these amplitudes,so that thenet position characteristic is a maximum positive value overone servo signal, decreasing to zero exactly halfway between the two,and increasing to a maximum negative value over the other servo signal.

In the present invention the servo signals are low frequency,linearly-recorded sine waves written in concentric circular tracks.Alternate servo tracks are written at different frequencies, the oddnumbered tracks at one frequency F1 and the even numbered tracks at asecond frequency F2 as shown in FIG. 2. Frequencies F1 and F2 should bechosen such that one is not a harmonic of the other. When the transduceris to the left of the geometric center of the distance between theadjacent servo tracks it will pick up signals at frequency F1 strongerthan at frequency F2. By filtering the frequencies F1 and F2 andseparately peak rectifying them, a low frequency DC. output signal isobtained whose amplitude varies as the transducer position relative tothe servo track position in a horizontal plane. When the transducer isin position directly over the geometrical center' between adjacent servotracks, it reads both F1 and F2 at the same amplitude and therefore theDC. output of F1 will equal the output of F2. If the two D.C. outputsare subtracted from each other, a transducer to track position outputprofile will be obtained similar to that of FIG. 3. Using this servotrack position output voltage curve, the transducer can be positioned tofollow the data tracks at the geometrical center of the distance betweenadjacent servo tracks.

One application of the present servo system is in connection with a dualmagnetic layer disk which includes two distinct magnetic layers ofdifferent coercivities superimposed on a non-magnetic substrate.Referring to FIG. 4, the cross section of such a disk is shown. Twomagnetic layers of different coercivities, an upper layer 42, and alower layer 43 are placed on substrate 41. In such an application, thelow frequency servo signals may be written in the lower layer and thehigh frequency data signals recorded in the upper layer directly overthe null point between adjacent servo tracks. The lower layer may bewritten permanently at a frequency or at a band of frequencies whoseupper limit is well below the lowest frequency contained in the upperlayer. All three signals (F1, F2 and data) can be read backsimultaneously by a single transducer and then separated by filtering.For the present servo system to be effective in connection with a dualmagnetic layer disk, the servo signals and the data signals must beindividually recognizable and one must exist independently of the other.The first requirement can be met by recording the data signals in theupper layer at a high frequency, at least three or four times thefrequency of the servo signals recorded in the lower layer. The secondrequirement can be met by providing the lower layer of the disk with aconsiderably higher coercivity than that of the upper layer. This willallow the servo signals to remain undisturbed by subsequent writing andrewriting of the data signals in the upper layer. The signals from bothlayers can be read simultaneously by a transducer having a single readgap as shown in FIGS. 5a and 5b. FIG. 5a shows a side cross section ofthe transducer having the erase head on the left, and the read-writehead on the right. FIG. 5b is a bottom view particularly pointing outthe single gap in both the erase head and the readwrite head. The singlegap in the read-write head is significant for purposes of having perfectalignment while simultaneously reading the control signals and the datasignals.

The amplitude subtraction involved in the present systern isaccomplished in the circuitry of FIG. 1. A wide band read amplifier 14amplifies all signals received from a single read gap in a readtransducer 15. A high frequency filter amplifier 16 is connected to theoutput of amplifier 14 to filter out the data signals. A pair of filteramplifiers 17 and 18 are connected to the output of amplifier 14. Eachof these filters is tuned to a particular frequency, 17 being tuned toF1 and 18 being tuned to F2, to detect the signals from the individualservo tracks. The respective servo signals are passed from amplifiers 17and 18 through a pair of linear gates 19 and 20, through A.C. amplifiers21 and 22 to peak rectifiers 23 and 24. The signal from amplifier 22 isinverted in peak rectifier 24, so that the two D.C. servo signals willbe of opposite polarity. This is accomplished by merely reversing adiode in peak rectifier 24. The peak rectified D.C. servo signals arethen applied to opposite sides of a D.C. summing amplifier 25 which addsthe two servo signals algebraically to provide a fine position errorsignal output for the transducer positioning motor or actuator 12. Sincethe two D.C. servo signals are of opposite polarity, the algebraic sumof the two will have the proper sign to provide the desired directionfor the fine position error signal. The linear gates 19 and 20 eachconsist of a pair of emitter followers which can be gated on and off topass or block the signal from its associated filter amplifiers 17 and18, respectively. Each of the filter amplifiers 17 and 18 is connectedto both linear gates 19 and 20. Since the position of servo tracks F1,F2 relative to odd numbered data tracks is reversed for even numbereddata tracks means is provided for reversing the connections between thefilter amplifiers and the linear gates to provide the proper directionsense for the position error signal from the summing amplifier 25. Thatis, for odd numbered data tracks amplifier 17 is connected to gate 19and amplifier 18 to linear gate 20, while for even numbered data tracksamplifier 17 is connected to gate 20 and amplifier 18 to gate 19. Thisreversal is carried out by a signal from the address register 13.Whether a track address is an odd number or an even number is indicatedby the presence or absence of a binary 1 in the least significant stageof the address register. A signal indicative of the load condition ofthis stage of the address register is transmitted to both linear gatesto control the emitter followers and gate the servo signals into theproper portion of the circuit.

The use of A.C. servo signals in the present invention permits A.C.amplifiers to be used in the circuitry of FIG. 1, with a consequent highdegree of accuracy in positioning the transducer to very closetolerances. With the circuitry of FIG. 1 the noise level becomes thelimiting feature. In fact, it is theoretically possible to accuratelyservo the transducer with this circuitry when the strength of the servosignal is at a one to one ratio with the noise level. With D.C.amplification this would be impossible because of the drift inherent insuch components.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in theform and details may be made therein without departing from the spiritand scope of the invention.

What I claim is:

1. A transducer positioning servo system for use in a random accessmagnetic disk memory including:

a magnetic recording disk having two magnetic layers of differentcoercivities superimposed on a non-magnetic substrate, the lower layerhaving a higher coercivity than the upper layer;

a plurality of closely-spaced, concentric, circular servo trackspermanently recorded on the lower magnetic layer in the form ofcontinuous linearly recorded sine waves, alternate ones of said servotracks being recorded at separate frequencies which are not harmonies ofeach other;

a plurality of closely-spaced, concentric, circular data tracks recordedon the upper magnetic layer, said data tracks being superimposed on saidservo tracks, the center line of said data tracks being over themidpoint between adjacent servo tracks;

a transducer for providing a continuous fine position signal bysimultaneously reading the sine waves of two adjacent servo tracks;

an actuator coupled to the transducer; and

circuitry connected to the transducer output for separating andcomparing two of the said sine waves on adjacent servo tracks, saidcircuitry developing a continuous servo position error signal for theactuator to position the transducer accurately onto the data trackpositions at the mid-point between adjacent servo tracks.

2. A transducer positioning servo system for use in a rotatable magneticstorage comprising:

a magnetic recording disk having a dual magnetic layer coating on anon-magnetic substrate;

a plurality of juxtaposed, concentric, circular servo tracks permanentlyrecorded on a first magnetic layer having a first degree of coercivity,said servo tracks being recorded in the form of low frequency sinewaves;

a plurality of closely-spaced, concentric, circular data tracks recordedon a second magnetic layer, said second magnetic layer beingsuperimposed over said first magnetic layer and having a lowercoercivity than the coercivity of said first layer, said data tracksbeing in the form of high frequency signals, said high frequency beingat least three times the frequency of the highest frequency servo signalrecorded in the lower layer;

a transducer, having a single read-gap, for simultaneously reading thesine waves on two adjacent ones of the said juxtaposed servo tracks onthe lower magnetic layer, and the data on one of the said data trackssuperimposed on said upper magnetic layer, thereby providing a compositesignal containing signals from all three of said tracks;

circuitry connected to the transducer output for separating saidcomposite signal, said circuitry developing a continuous servo positionerror signal; and

an actuator coupled to the transducer and responsive to the polarity ofsaid error signal for accurately positioning the transducer over thenull point between two adjacent ones of the said juxtaposed servotracks, thereby positioning said transducer over the center line of adesired one of the said data tracks.

3. A transducer positioning servo system for use in a random accessmagnetic disk storage comprising:

a magnetic recording disk having a dual magnetic layer coating on anon-magnetic substrate;

a first magnetic layer having a relatively high coercivity superimposedon said non-magnetic substrate, said high coercivity layer being adaptedfor having recorded thereon a plurality of low frequency servo tracks;

a second magnetic layer, having a relatively low coercivity on saidfirst magnetic layer, said second magnetic layer being adapted forhaving recorded thereon a plurality of high frequency data tracks;

a transducer having a single readgap for simultaneously reading signalsfrom both said first and said second magnetic layers, the signal fromthe first layer comprising two of the said low frequency servo tracks,the signal from the second layer comprising one of the said highfrequency data tracks;

circuitry connected to the transducer output for separating said threesignals and comparing the two signals from said first magnetic layer,said circuitry developing a continuous servo position error signal;

an actuator coupled to the transducer and responsive to said errorsignal for positioning the transducer accurately over a null pointbetween two of the said low frequency servo tracks, thereby maintainingsaid transducer over the center line of a desired one of the said datatracks.

4. A transducer positioning servo system for a random access magneticdisk storage comprising:

a magnetic recording disk having two magnetic layers of diiferentcoercivity superimposed on a non-magnetic substrate;

a plurality of juxtaposed, concentric, circular servo tracks recorded onthe lower magnetic layer in the form of continuous linearly recordedsine waves, adjacent ones of said servo tracks being recorded atseparate frequencies which are not harmonics of each other;

a plurality of closely-spaced, concentric, circular data tracks recordedon the upper magnetic layer, the center line of said data tracks beingover the boundary line between two of the said juxtaposed servo tracks;

a transducer, having a single read-gap, for simultaneously reading thesine waves on two adjacent ones of the said juxtaposed servo tracks onthe lower magnetic layer, and the data on one of the said data trackssuperimposed on said upper magnetic layer, the transducer providing acomposite signal containing signals from all three of said tracks;

circuit means connected to the transducer output for separating andcomparing the two sine waves on adjacent servo tracks, and fordeveloping a continuous servo fine position error signal; and

an actuator coupled to the transducer and responsive to the polarity ofsaid error signal for accurately positioning the transducer over saidboundary line between two adjacent ones of the said juxtaposed servotracks.

References Cited UNITED STATES PATENTS 2,643,130 6/1953 Kornei 179100.22,647,954 8 /1953 Howell 179-1002 2,714,133 7/1955 Barry 179-10023,052,567 9/1962; Gabor et a1. 179l00.2 3,185,775 5/1965 Camras 179-40023,263,031 7/1966 Welsh 340-174.]

BERNARD KONICK, Primary Examiner. V. P. CANNEY, Assistant Examiner,

